1. Field of the Invention
Embodiments of the present invention generally relate to a thin film transistor (TFT) and a method of its manufacture.
2. Description of the Related Art
Current interest in TFT arrays is particularly high because these devices may be used in liquid crystal active matrix displays (LCDs) of the kind often employee for computer and television flat panels. The LCDs may also contain light emitting diodes (LEDs), such as organic light emitting diodes (OLEDs) for back lighting. The LEDs and OLEDs require TFTs for addressing the activity of the displays.
Bottom gate TFTs made with amorphous silicon have been utilized for the flat panel display industry for many years. Unfortunately, the on and off-current driven through source and drain electrodes of the TFT is limited by its channel material as well as the channel width and length. Additionally, the turn-on gate voltage, or a threshold voltage, under a voltage between the source and drain electrodes, is determined by the accumulation of the carrier in the active channel area of the semiconductor active layer which could change as the change of the charge in the channel material, dielectric material as well as interfaces between the materials after bias temperature stress or current temperature stress.
Therefore, there is a need in the art for a TFT that utilizes an additional source-drain current controlling layer beneath the source or drain electrode or both to miminize the off-current when the gate voltage sets at a turn-off voltage. Because the additional control of the source-drain current control, the channel length between the source-drain can be significantly reduced for high on-current when the TFT turns on, keeping the off-current low when the TFT turns off. In addition, the TFT design keeps the turn-on voltage, or threshold gate voltage, unchanged after operations under different conditions.